Neutral Atomic Gas Research Articles

Infrared observations of the center of the Galaxy

We report velocity resolved maps of the 158 μm [CII] fine structure line, of the 63 μm [OI] fine structure line, and of the 88 μm and 52 μm [OIII] fine structure lines toward the center region of the Galaxy. Observations were carried out on board the Kuiper Airborne Observatory (KAO) using the MPE/UCB Far-infrared Imaging Fabry-Perot interferometer (FIFI). The angular resolution was 55″ for the [CII] line and 22″ for the oxygen lines. The [CII] emission, tracing the distribution of dense gas dissociated and partially ionized by far-UV photons, is strongest toward the rotating circumnuclear disk surrounding Sgr A West. A continuous bridge of [CII] line emission connects the Sgr A complex to the thermal radio filaments in the Radio Arc 10' north of the center, thus strongly suggesting a direct physical connection between the two. Outside the nuclear region, there is an anticorrelation between the [CII] intensity and the distribution of dense molecular material. The brightest emission occurs near the edges of the massive Galactic Center molecular clouds. We conclude that these clouds are predominantly ionized by external UV photons, partly from the Sgr A West region and partly from OB stars near the center. Our data indicate a physical connection between the center and the massive interstellar clouds in the surrounding 50 pc. Our 88 μm [OIII] maps of the “sickle” region within the Radio Arc and of the thermal arches approximately follow the thermal radio continuum. The line width varies substantially over the mapped area; we find a narrow ( Δv ≈ 60 km/ s) and a broad ( Δv ≈ 150 km/ s) component, with the broad component being more prominent in the sickle. From the 52 μm 88 μm line ratio we derive a density ≈ 300 cm −3 and a hydrogen column density ≈3 × 10 21 cm −1 in the [OIII] emitting medium. This rules out shocks or fast winds from stars as excitation mechaniμs. UV excitation, however, could produce the observed amount of O 2+. About 10 to 100 O-stars would be required to ionize the gas in the “pistol/sickle” region. The broad linewidth could be due to interaction between the ionized gas and magnetic fields. Our maps of both [OIII] lines toward the circum-nuclear disk show the brightest emission from inside the central, “ionized cavity” in the molecular ring. The high 52 μm/88μm ratio requires a density ⩾ 10 4 cm −3. The derived hydrogen column density, however, is low, which could be understood by a small ionization ratio N o2+ N o0 or a small beam filling factor of the ionized gas. We infer about 10 m⊙ of ionized gas inside the cavity, with no indication of an additional low-density component. The [OI] data, on the other hand, reveal the presence of a large amount of neutral atomic gas inside the central cavity, and associated with the “northern” and “eastern” arms of ionized gas. We estimate about 300 M⊙ of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuelear disk and falling from a gap in the 1.5pc ring into the central 0.5pc. We infer a mass inflow rate into the central parsec of 0.03 M⊙/year.

Internal ion impact ionization for Fourier-transform ion cyclotron resonance

A general scheme for ionization and fragmentation of ions in Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICRMS) is introduced. The technique is based upon excitation of high-frequency (i.e. lowmass) primary ions (e.g. N, Al+, H2O+·) which may be generated by standard ionization methods (e.g. electron ionization or laser desorption) inside the trap. Generation of the primary ions is followed by excitation of their cyclotron motion to high translational energy to serve as projectile ions. The projectiles serve two functions: to ionize neutral gas atoms and/or molecules (secondary ions) within the trap internal ion impact ionization (IIII), and to collisionally activate the secondary ions to induce fragmentation. Ionization by IIII is analogous to charge-exchange ionization, in which a primary ion reacts with neutral analyte to produce ionization. When the projectile ions' cyclotron orbits are not excited (even for very long delay periods between ionization and detection), lower ion abundances are observed for the secondary ions derived from the neutral analyte(s), verifying that high energy ion impact must be responsible for the relatively high ion abundances observed when projectile cyclotron motion is excited: i.e., the mechanism of IIII is likely to be high-energy charge exchange. Several advantages of the new technique include: accessibility of very high impact energy (keV) in FT-ICRMS in both the laboratory and the center-of-mass frames of reference; ease in controlling the average energy transferred from projectiles to sample neutrals by moderation of the projectile ion's cyclotron radius; and the high ion-molecule reaction rates attainable in FT-ICR due to the long path length for trapped primary projectile ions at low pressure. Theory of ion formation and the problems associated with this technique (e.g. formation of secondary ions at different pre-excitation ICR orbits) are discussed.

Computational Studies on the Shape and Control of Plasmas in Magnetron Sputtering Systems

The three-dimensional motions of electrons are traced in magnetron sputtering systems. Electrons are emitted from a cathode and show drift motion. Collisions between electrons and neutral gas atoms of Ar are examined using the Monte Carlo method. The distribution of ionization points is used to show the shape of plasmas. The results of the simulations are as follows. The profiles of plasma distribution correspond to the shape of the magnetic field. The shape of erosion profile caused by sputtering agrees well with the plasma profile. The increase of ionization points is obtained by increasing the neutral gas pressure.

Empirical and semiempirical interaction potentials for rare gas–rare gas and rare gas–halide systems

The Tang–Toennies (TT) semiempirical model potentials for ion–atom systems is applied to the rare gas–halide negative ion exciplexes. The coefficients defining the repulsive Born–Mayer term in the TT semiempirical potentials are determined from the equilibrium bond length Re and dissociation energy De taken from ab initio calculations and from transport studies of these molecular ions. The damped dispersion and induction energy terms in the TT potentials are obtained from coupled Hartree–Fock calculations for the neutral rare gas atoms and F− and Cl− ions. The multipole polarizabilities for the heavier halogen atomic negative ions are estimated from a knowledge of polarizability ratios across isoelectronic sequences. The resultant semiempirical ionic potentials are compared to available ab initio calculations and the results of inversion of transport theory. To facilitate the comparison of the (sparse) ab initio data with the semiempirical potentials, a simple fitting procedure is presented for determining empirical potentials for diatomic molecules from a set of three constraint equations. The fitting procedure is applied to a total of 22 rare gas excimers and rare gas–halide exciplexes (both neutral and ionic) of interest to a variety of applications in gaseous discharges and excimer lasers. A three-term representation of the empirical potentials generated is accomplished with the use of a minimal data set which include the ‘‘geometric’’ parameters {R0,Re,De} and the additional parameters {αd, I.P., E.A.} needed for the dispersion and induction energy terms. A novel feature of the empirical procedure is the formulation of the constraint equations at two nuclear displacements (one constraint at R0, wherein the potential passes through zero, and two constraints at Re the equilibrium separation) which yields an accurate fit to available ab initio data and greatly extends the range of internuclear separations R for which an accurate piecewise analytical empirical potential can be generated. To test the relative importance of the different terms in the fitted three-term empirical representations, the classical orbiting cross section Qorbit(E) is computed using the full empirical potential and compared against the standard Langevin orbiting cross section Qpol(E) for a pure polarization interaction.

Removing the energetic neutral problem in sputtering

The evidence for the reflection of neutral inert gas atoms from the target being bombarded during sputter deposition is reviewed. This evidence includes theory, effects at the target, such as measurements of the fraction of incident energy which enters the target from bombarding ion beams, and effects at the growing film, such as the heating effect, the production of stress, the emission of secondary electrons, and the burial of inert gas atoms. New results are reported for the burial of energetic neutral atoms in simple metals in discharges in argon, krypton, and xenon. These results confirm the presence of a high flux of highly energetic neutrals when the sputter atom is substantially less massive than the target atom, leading to the inert gas being included substitutionally at high concentrations in the growing film. To achieve high purity in sputtered films with heavy atomic materials, xenon rather than argon should be used as the sputter gas. A rethink of sputter plant design shows that this expensive gas can be used economically, with some interesting consequences. The performance of a small system based on this rethink is described.

H I streamers around M82 - Tidally disrupted outer gas disk

Our new VLA observations of M82 reveals a triangular central concentration of neutral atomic gas (1.9×10 8 M ○. ) and several ≥10 kpc long tidal streamers originating from M82 itself. The coincidence of the beginning of the tidal streamer with the flaring of the optical disk as well as the smooth continuation of the disk velocity field into the H I streamer strongly suggest the tidal disruption of the disk of M82, contrary to the widely proposed scenario of tidal stripping and gas accretion from its neighbor M81. The velocity characteristics of H I within M82 are that of disk rotation as seen in CO. The gas kinematics appear complex because of large velocity dispersions that may be due to a tidally induced bar potential and the interaction between the disk gas and the nuclear wind

Imprints of the interstellar medium on location, geometry and nature of the heliospheric shock

The fact of a relative motion of the solar system with respect to the ambient interstellar medium causes a series of qualitatively very different physical influences on both the supersonic and the subsonic solar wind plasma regimes. We are quantifying some of these influences here which are connected with the action of interstellar neutral gas atoms, low and high energy plasmas and magnetic fields. As is shown quantitatively here the distribution of neutral interstellar gases in the heliosphere, and the interplanetary UV/EUV glows and pick-up ion fluxes connected with it, have received clear imprints from the ionized component of the interstellar medium which, when carefully analysed, will permit the derivation of many unknown parameters of this component. From our synoptic analysis of distant upwind and downwind interplanetary Lyman-Alpha data we shall conclude that these data can only be reconciled under the strong plead for a termination shock at an upwind distance of about 50 to 70 AU. No clearcut answer, however, can be given as to why the shock is forced to be located such close to the sun. Furthermore we derive theoretical arguments why the heliospheric shock and the supersonic solar wind approaching it should have pronounced upaxis/downaxis asymmetries which when clearly determined could reveal a tilt of this axis with respect to the neutral interstellar wind vector and thereby give some clear hints towards the magnitude and the direction of the interstellar magnetic field. Starting from theoretical solutions for the postshock plasma flows which are consistent with the flow of the ambient interstellar medium we derive the location and geometry of the solar termination shock converting an inner supersonic solar wind to this postshock flow.

Charge transfer and collision-induced dissociation reactions of CO++ with the rare gases at Elab=49 eV

Multiple product channels are observed for the reaction of 13CO++ with each of the rare gases (Rg) at Elab=49±1 eV. A beam of 13CO++ is produced by electron impact ionization and is mass selected using a quadrupole mass spectrometer. The ion beam is focused into a collision region and the reaction products are monitored using time-of-flight mass spectrometry. Relative yields for the production of 13C+, O+, and 13CO+ are measured directly. Absolute charge transfer reaction cross sections for collisions of 13CO++ with He, Ne, Ar, and Kr are estimated by comparing the Rg+ production with that for the charge transfer reactions of doubly charged rare gas ions with neutral rare gas atoms. The cross sections are found to range from 0.9−0.9+1.5 Å2 for collisions of 13CO++ with He to 37.5±19.6 Å2 for collisions with Kr. The reaction of 13CO++ with He proceeds almost exclusively into the collision-induced dissociation channel. The branching fraction for collision-induced dissociation is smaller for reactions with Ne and almost disappears for Ar, Kr, and Xe. As the relative importance of the collision-induced dissociation process decreases, branching into the charge transfer channel increases. The charge transfer reactions of 13CO++ with Ar, Kr, and Xe are shown to populate excited, dissociative electronic states of 13CO+ selectively. These effects are modeled successfully using Landau–Zener theory in conjunction with reaction window theory.

Simulations on Electron Orbits in Magnetron Sputtering Systems.

The motions of electrons are traced in magnetic and electric fields of magnetron sputtering systems.Electrons are emitted from a target cathode and show drift motions resulting in generation of plasmas. Collisions between electrons and neutral gas atoms of Ar are examined by using Monte Carlo method.Ionization points are used to calculate sputtering rate on a target. The calculated profiles of sputtering rate show good agreement with the erosion profiles measured in experiments.

Ionized carbon in the Large Magellanic Cloud

The 158 micron 2P3/2-2P1/2 fine-structure transition of C(+) at selected locations in the LMC. The C II emission is most intense toward far-infrared continuum peaks and generally is not seen in positions exhibiting strong CO J = 2-1 radiation. Where both C II and CO emission are detected, the V(LSR) centroids are similar but the C II line is wider. The differences in spatial distribution and spectral shape suggest a more pronounced physical separation between the predominantly neutral atomic and molecular gas regions than is the case in the Galaxy. In the LMC, the intense and extended C II emission near 30 Dor implies a total amount of C(+) several times greater than that of Galactic molecular cloud complexes. An attempt was made to detect the 289 micron J = 9-8 transition of (C-12)O in a few locations. The observed upper intensity limit for N159 implies that moderate density molecular gas fills less than 5 percent of the beam and that most of the low J CO emission comes from lower density gas.

Effects of sputter gas medium on the nanometer-scale surface structures of the Pt/Co multilayers

The surface structure of polycrystalline Pt/Co multilayer thin films, sputter-deposited on glass substrates with Ar, Kr, or Xe, was obtained with near-atom resolution by scanning tunneling microscopy. Monatomic steps separating narrow terraces on the Ar-sputtered films and large step-like structures on the order of the Pt/Co bilayer thickness on the Xe-sputtered films were observed. We attributed differences in surface features to bombardment of the growing film surface by reflected neutral inert gas atoms with different energy distributions. We correlated the observed surface structures with magnetic coercivity of these films.

A model for negative ion extraction and comparison of negative ion optics calculations to experimental results

Negative ion extraction is described by a model that includes electron diffusion across transverse magnetic fields in the sheath. This model allows a two-dimensional approximation of the problem. It is used to introduce electron space charge effects in a 2-D particle trajectory code, designed for negative ion optics calculations. Another physical effect, the stripping of negative ions on neutral gas atoms, has also been included in our model; it is found to play an important role in negative ion optics. The comparison with three sets of experimental data from very different negative ion accelerators, show that our model is capable of accurate predictions.

Formation of cosmic structure by Doppler instability

A new mechanism is described which can create an instability in homogeneous gaseous matter at very low density. When an isotropic background radiation field has, near an electronic resonance (such as the hydrogen Lyman-α line), a spectral feature for which photon occupation number increases with frequency, moving atoms increase their speed by taking energy from the photon distribution. In a cosmological setting, a sufficiently intense spectral feature can interact with neutral atomic gas, after recombination, to generate protogalactic perturbations of the scale and magnitude needed to explain large-scale cosmic structure.

Semiempirical study of rare gas and rare gas–hydrogen ionic clusters: R+n, (RnH)+, and (RnH2)+ for R≡Ar, Xe

The ionic rare gas clusters Ar+n and Xe+n and rare gas–hydrogen clusters (ArnH)+, (ArnH2)+, (XenH)+ and (XenH2)+ are studied by the semiempirical diatomics-in-ionic-systems (DIIS) method. The Ar+n clusters (n>3) are seen to have a structure of a linear Ar+3 core surrounded by n−3 neutral or almost neutral Ar atoms. For Xe+n (n>3), a symmetrical Xe+4 ionic core with the geometry of regular pyramid is formed. The rare gas–hydrogen clusters with one H atom have a simple Rk(RH)+ structure with k neutral rare gas atoms attracted to the (RH)+ molecule by polarization forces. Two H atoms can bind with Ar atoms to form quasistable clusters ArnH+2 which dissociate to (n−1)Ar+H+(ArH)+ through a high barrier of roughly 0.75 eV. Two H atoms and one Xe+ ion are shown to form a collinear valence-bound (XeHH)+ cluster whose dissociation energy is 0.46 eV.

Recent results from photon stimulated desorption of rare gas solids excited with synchrotron radiation

Desorption of neutral rare gas atoms following selective excitonic excitation of rare gas solids (Ne, Ar, Kr, Xe) by VUV photons underlines the crucial role of excitons concerning (i) transport of excitation energy from the bulk to the surface, and (ii) the microscopic desorption mechanism itself. All rare gas solids yield desorption of ground state atoms which stem from relaxation of molecular-type excitons after their radiative decay. In addition, solid Ne and Ar yield desorption of 3P1, 1P1, and metastable (3P2, 3P0) atoms. Total desorption yields as well as partial yields of excited atoms will be presented. The dominating role of primary excitation of surface excitons is obvious for excited atoms. Primary creation of free electron-hole pairs favours desorption of ground state atoms. The kinetic energy distributions of metastable Ne and Ar atoms were determined by a time-of-flight technique. Maxima were found at ≅ 0.2 eV (Ne) and 0.04 eV (Ar) which establish a new desorption mechanism of excitonic origin.

Stresses in Amorphous Gd-Fe Alloy Thin Films Deposited by Magnetron Sputtering

ABSTRACTAmorphous Gd-Fe alloy thin films were made by D.C. planar magnetron sputtering under various deposition conditions (e.g., film thickness, composition, working pressure of Ar, negative bias voltage and deposition rate). The stress, the film composition and the content of entrapped Ar in the films were measured respectively. The experimental results showed that in this case the working pressure of Ar and the negative bias voltage did not change the composition of the films, and the stresses were all compressive except for the films deposited in a very high working pressure of Ar. The origin of the compressive stress can be attributed to the atomic peening effect produced by fast neutral working gas atoms rebounded from the sputtering target. The magnitude of the compressive stress depends not only on the amount of Ar atoms incorporated in the films but also on the film microstructure such as the packing density.

Infrared Emission at High Galactic Latitude

Results obtained with the Infrared Astronomy Satellite (IRAS) on the IR emission at high galactic latitude are reviewed. We present evidence for the detection of galactic emission at 12, 25, 60, and 100 μm. We describe the morphology of this emission and summarize work on the correlation of IR cirrus with H I, CO, and optical emission. We discuss the contribution of the neutral atomic, ionized, and molecular gas to the total IR emission and the contribution of different components of interstellar dust to the emission seen at different wavelengths.

Storage rings for spin-polarized hydrogen

A strong-focusing storage ring is proposed for the long-term magnetic confinement of a collisional gas of neutral spin-polarized hydrogen atoms in the |a〉 and |b〉 hyperfine states. The trap uses the interaction of the magnetic moments of the gas atoms with a static magnetic field. Laser cooling and evaporative cooling can be utilized to enhance the confinement and to offset the influence of viscous heating. An important application of the trap is to the attainment of Bose–Einstein condensation.

HI Absorption Measurements over the Galactic Centre Radio Arc and Arches Regions

An HI absorption study of the central ~ 100 pc of the galactic centre was performed using the compact array of the VLA2. The results of these observations lead us to believe that the body of gas at 40–50 km s−1, known as the ‘40 km s−1molecular cloud’, is placedbehindthe Arc. In addition, the distribution of gas around +20km s−1across the Arc and its correlation with 3cm polarized emission suggests that it may be this 20km s−1gas which is associated with the Arc and is depolarizing much of its synchrotron radiation. The kinematic structure of the negative velocity HI gas across the arched filaments implies an association between the neutral atomic gas and the ionized and molecular material.

Optical emission spectroscopy for analysis of broad ion beams

In sputter etching and reactive ion beam etching (RIBE), contamination and disturbance free in situ methods are desired for control of etch relevant quantities like energy and angular distribution of particles. These requirements cannot be fulfilled by the conventional electrical methods. In this paper we show that optical emission spectroscopy is a valuable tool for in situ diagnostics of broad ion beams. We report on measurements in Argon ion beams of 300–1000 eV energy. Several emission lines of fast and slow atoms and ions are detected in the wavelength range 4000–7000 Å at a resolution of better than 0.1 Å. Impact of fast particles and electrons on neutral background gas atoms are specified as main excitation mechanisms. From the Doppler-shifted emission lines we obtain information on energy and angular distribution of the beam particles.